Fiber optic cables have a number of advantages relative to electrical conductor, such as copper wire, for transmission of communication signals. Optical fibers can carry much more data than can a similar size electrical conductor. Very importantly, optical fiber, unlike electrical conductor, is not subject to electromagnetic interference, a feature particularly important when carrying data such as computer signals. Optical cable for data transmission has immediate importance in transmitting communication signals locally, e.g., between one computer or word processing terminal and another. Also, fiber optic cables are considerably lighter than electrical cables.
Localized communication cables extend through buildings, typically through plenum regions between floors of large buildings, and frequently also extend from building to building. It is important that a cable which passes through building plenums or the like have low smoke-producing and low flame-spreading properties. For cables that are to be used in construction in the United States, it is generally required that they meet National Electrical Code requirements pertaining to the smoke-producing and flame-spreading properties of the cable.
At the same time, it is important that the optical cable transmits the optical signal without significant attenuation. Signal attenuation is a particularly significant problem with optical cables that transmit signals through plenum regions with a wide range of temperatures. A communication cable may pass through inside but unheated areas, outside regions where it is subjected to winter temperatures, and at the same time, it may pass through regions in close proximity to heating pipes or the like. Thus, it is considered desirable that an optical cable be stable over a wide range of temperatures, and for a plenum cable, thermal stability preferably includes the temperature range of from -40.degree. C. to 80.degree. C.
Optical fibers consist of a central glass core, through which the light rays are actually transmitted, and means to retain the light within the central core, such as a surrounding cladding having a lower refractive index than the core so that a core-cladding interface tends to reflect rays back into the core rather than penetrate the barrier to become lost from the optical fiber. The transmittance of the optical fiber depends to a large extent upon the uniformity of the core-cladding interface. Light transmitting through an optical fiber travels in different modes, that is, at differing angles with respect to the axis of the core. Lower order light modes pass through the fiber at minimum angles with respect to the core axis, striking the core-cladding interface at low incident angles and reflecting back into the core. Higher order light modes pass through the fiber at greater angles with respect to the axis of the core, and hence strike the interface at greater incident angles and also travel a greater total distance through the fiber. These factors contribute to higher order light modes being relatively quickly lost from the fiber while lower order modes may pass through a substantial fiber length without significant attenuation. The light transmission attenuation of an optical fiber is a function of the uniformity of the core-cladding interface because distortions in this interface generate more easily attenuated higher order light modes from lower order light modes.
Light attenuating distortions in the core-cladding interface may arise if the cable's optical fibers are subjected to differential stress throughout their length. Differential stresses on the fibers may arise when the cable is subjected to wide temperature variations throughout its length as a result of differential thermal expansion and contraction of the various materials of which the cable is formed according to their various coefficients of thermal expansion. The differential stresses may either be radial, as a result of surrounding cable material pressing inwardly differentially on the optical fibers, or longitudinally, as a result of surrounding material expanding or contracting differentially relative to the optical fibers. For a cable which is to be subjected to wide temperature swings throughout its length, it is desirable to isolate the optical fibers from the effects of differential expansion and contraction of the materials as much as possible to minimize attenuation of light transmitted through the fibers.